Thermoplastic copolyetherester elastomers form a highly useful class of polymers because of their outstanding physical properties and excellent processing behavior. However, it is known that the copolyetheresters in contrast to polyesters are highly susceptible to oxidative degradation induced by exposure to heat or light. Thermally induced oxidative degradation is described in a paper by G. K. Hoeschele, Angewandte Makromolekulare Chemie, 58/59, pps. 299-319 (1977), which reports complete degradation of unstabilized copolyetheresters within 24 hours at 121.degree. C. The rapidity of light induced oxidative degradation is evidenced by the complete failure of 0.25 mm films of copolyetheresters after about 6 weeks exposure indoors behind glass to northern sky light.
The classical solution to instability problems of the type exhibited by copolyetheresters has been to add a combination of an antioxidant and a UV-absorber. Such measures generally have been found to be adequate in related polymers such as polyether-based polyurethanes. In contrast to the polyurethanes, copolyetheresters respond to only a limited extent to the combination of antioxidant and UV-absorber. This contrasting behavior is further evidence of the unusual susceptibility of copolyetheresters to oxidative degradation.
Relatively recently, a new class of highly effective photostabilizers, characterized by the presence of hindered piperidine moieties, has become available. A number of such photostabilizers have been evaluated in copolyetheresters where their performance as photostabilizers is good. Japanese Patent Application Publication No. 75/91652 discloses the use of a number of hindered piperidine type photostabilizers in combination with phenolic antioxidants in copolyetheresters. It has been found that when one follows the teaching therein, photostabilization of the copolyetherester is indeed improved; however, heat-aging behavior is much poorer when the photostabilizer is present compared to performance in the absence of the photostabilizer. This deficiency cannot be corrected by simply increasing the concentration of phenolic antioxidant.
An additional problem encountered on many occasions is lack of compatibility of the photostabilizer at higher concentrations with the copolyetherester which results in migration of the photostabilizer to the surface of the polymer with the formation of bloom. Also, conventional photostabilizers can be extracted from or volatilized from the polymer in certain environments which reduces the stability of the substrate.
Thus, a need for copolyetherester compositions which simultaneously provide outstanding heat and light aging performance without the problems of prior art stabilizers still exists.